UN 1638 — Mercury iodide
Placard: Toxic. ERG Guide 151. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
UN 1638 is Mercury iodide, a toxic mercury compound assigned to ERG Guide 151. It is usually a solid salt where dust, solution and runoff contamination are the major responder concerns.
Hazard overview: UN 1638 presents toxic mercury dust, ingestion, skin contact and contaminated-runoff hazards. Heating or fire may release mercury-containing fumes, and dry powder can contaminate clothing and equipment.
Response guidance: For a UN 1638 incident, responders should verify the product with shipping papers, package markings, SDS and ERG Guide 151. Establish incident command, isolate the area, stay upwind, prevent dust or vapor exposure, control runoff and choose entry or cleanup actions based on monitoring, SDS and local SOP.
Firefighter training notes: Training for UN 1638 should emphasize mercury salt toxicity, dust or solution contamination, SCBA use, runoff containment, decontamination and waste-control coordination. Use ERG 151, SDS and local SOP.
Regulatory context: Mercury iodide is regulated as a toxic mercury hazardous material. Transportation, occupational exposure, spill reporting, waste handling, storage and environmental controls may vary by compound, quantity and jurisdiction. Verify current requirements through shipping papers, SDS, facility documents and applicable DOT, OSHA, EPA, NFPA, state or local authority guidance.
Storage & handling: Mercury iodide should be stored in tightly closed compatible containers in a secure, cool, dry, well-ventilated toxic-material area away from food, incompatible chemicals, heat and unauthorized access. Prevent dust release, solution leaks and mercury-contaminated runoff.
UN 1638 Quick Details
Common Hazards of UN 1638
- HIGHLY TOXIC mercury compound; may be harmful or fatal if inhaled, ingested or absorbed through skin.
- Dust or solution can contaminate clothing, tools, equipment and runoff.
- Fire or heating may produce toxic mercury fumes and irritating decomposition products.
- Runoff may pollute waterways and create persistent contamination.
- Containers may rupture or fail when heated.
- Specific toxicity, solubility and reactivity depend on the exact mercury compound.
- Avoid skin contact and prevent dust from becoming airborne.
Chemical Identity & Physical Properties
Odorless red to yellow crystalline powder or solid. Scarlet red crystals are the most common form at room temperature.
| Also known as | Mercuric iodideMercury(II) iodideRed mercury iodideHgI2Biniodide of mercury |
| CAS Number | 7774-29-0 |
| Appearance | Odorless red to yellow crystalline powder or solid. Scarlet red crystals are the most common form at room temperature. |
| Flash Point | Not applicable (non-combustible solid) |
| Boiling Point | 354C (669F) with decomposition |
| Vapor Density | Not applicable (solid) |
| Water Reactivity | Insoluble in water; no significant reaction |
Fireground Response Guidance — UN 1638
Extinguishing Media
PPE Requirements
Use positive-pressure SCBA for dust, vapor, fire or confined-space exposure. Chemical-resistant gloves, eye/face protection and protective clothing should be selected from SDS; prevent skin contact and mercury-contaminated dust or runoff spread.
Isolation & Evacuation
First Actions for a UN 1638 Incident
- CALL 911. Then call the emergency response telephone number on the shipping paper, if available.
- Keep unauthorized personnel away.
- Stay upwind, uphill and/or upstream.
- Avoid breathing vapors, fumes, dust or mist and avoid all skin or eye contact.
- Do not touch or walk through spilled material unless properly trained and wearing appropriate protective equipment.
- Avoid creating dust clouds or spreading contaminated powder, solution, runoff or debris.
- Ventilate closed spaces before entering, but only if properly trained, equipped, monitored and authorized by incident command.
- Isolate the spill or leak area and expand the perimeter if dust, vapor, fire involvement or unknown concentration is present.
- Use ERG Guide 151, shipping papers, SDS, air monitoring and incident command for protective actions.
📋 Copy & Share Field Card
UN 1638 — Mercury iodideUse for: Quick radio or face-to-face size-up. Short, structured, field-ready.
Use for: Incident command briefing, staging area whiteboard, or pre-entry team brief.
Use for: Quick text to command or incoming units. Fits in a single SMS.